Load frames are used in a variety of applications, in particular to apply forces to a test specimen. This allows strains to be measured on the test specimen and evaluated to determine if the material of the test specimen will be suitable for an intended application. Prior to analyzing the test specimen, however, the load frame has to be checked and calibrated with an alignment specimen to ensure that the force that will be applied to the test specimen is not angularly or concentrically misaligned with an axial center of the test specimen.
Alignment of the load frame is accomplished by securing an alignment specimen between an upper head and a lower head of the load frame. The lower head is secured to a load bar which is movable linearly vertically on a pair of parallel rails. The upper head is fixed at a desired position on the load frame, and thereafter cannot move. The alignment specimen is secured at its opposite ends between the upper and lower heads. The alignment specimen typically includes a plurality of strain gages secured to it at spaced apart locations along its length. The strain gages provide indications of the bending force experienced by the alignment specimen at various points along its length as a result of the force applied to it by the lower head. If the lower head is angularly or concentrically misaligned, then the force that is applied by it will not be applied directly along the axial centerline of the alignment specimen. Thus, the alignment specimen will be subjected to bending forces at one or more point(s) along its length. An improperly aligned load frame will produce inconsistent test data when the test specimen is placed in the load frame and is subjected to a controlled force. This can lead operators to improper conclusions about the properties of the materials being tested.
Presently available load frames are typically adjusted to remove angularity and/or concentricity misalignments of the lower head relative to the upper head by collecting data from the instrumented alignment specimen. The data is then displayed on a graph to aid the operator at making alignment adjustments. Often the graph is an X-Y graph which attempts to represent the bending condition(s) at one or more points along the alignment specimen in two dimensional fashion. Since the adjustments to the load frame are made independent of each other (i.e., concentric and angular adjustments are made independently), a key problem is to determine whether the bending being experienced at a given point on the alignment specimen is due to a concentric or angular misalignment of the lower head relative to the upper head. In practice, adjustments have been made for angular and concentric misalignments by trial and error from information presented on X-Y graphs (i.e., in two dimensional fashion). This is often a time consuming process, and it typically needs to be repeated every time new test hardware is used in a load frame.
Accordingly, it would be highly desirable to provide an apparatus and method that more easily, and intuitively, allows the operator to determine and correct for angular and concentric misalignment of the lower head of a load frame relative to the upper head.